Bile pigments in plants and animals are structurally related by virtue of their common origin from the carbon skeleton of protoporphyrin IX, but are otherwise distinct in at least two fundamental ways. First, plant bile pigments serve as photoreceptors for a host of light mediated processes necessary for ontogenetic development of the organism, whereas mammalian bile pigments are transient products of hemoglobin catabolism with no known physiologic function. Second, plant bile pigments are covalently linked to specific cellular proteins and the resulting chromophore-protein interaction renders them insensitive to photo-oxidation in vivo. In contrast, mammalian bile pigments form non-covalent bonds with proteins (eg., albumin, ligandin) and are susceptible to destruction in high intensity light. The overall objectives of the proposed project are to (1) determine the mechanism of bile pigment formation in plants, (2) determine the structure of the plant protein to which plant bile pigment is covalently attached, and (3) determine the properties (physical-chemical) of the bile pigment-apoprotein interaction which render plant bile pigment stable in the light.